Our invention relates generally to vehicular headlamps and more specifically to those suitable for use on motorcycles or like vehicles that tilt laterally as when taking a curve or bend in a road. Still more specifically, our invention pertains to a system for use with such vehicular headlamps for automatically maintaining the pattern of the light beam, particularly the lower beam, thrown thereby in a level position about the beam axis in the face of variations in the lateral angle of the vehicle with respect to the road surface.
Vehicle headlamps today are usually constructed to provide the so called "upper beam" and "lower beam". The upper beam is intended primarily for distant illumination when the vehicle is not meeting or following other vehicles. The lower beam is intended to illuminate the road ahead of the vehicle when it is meeting or following another vehicle. The pattern or cross sectional shape of the lower beam, in particular, is definitely determined (see FIG. 10 of the drawing attached hereto) for the dual purpose of providing optimum road illumination for safe driving and of protecting other drivers from glare.
A problem has been encountered in connection with such two beam headlamps on motorcycles or like two wheeled vehicles. The vehicles of this class must tilt laterally when taking a curve or bend in a road, in order to avoid being centrifugally carried off the road or toppling over. As the vehicle tips in either of the opposite lateral directions, so does the pattern of the lower beam emitted by its headlamp. Thus angularly displaced from its level position, the lower beam pattern fails to perform its intended functions, illuminating the road either too close to, or too far away from, the vehicle, or not illuminating its lane at all, and possibly dazzling the drivers of oncomming vehicles.
Miyauchi et al. U.S. patent application Ser. No. 07/137,021, pending filed Dec. 22, 1987, represents an attempted solution to the above stated problem. This prior application teaches a motorcycle headlamp unit wherein the bulb and the control lens are rigidly interconnected for joint bidirectional rotation, within limits, about the lamp axis with respect to the lamp housing, thereby making possible the angular displacement of the beam pattern about the lamp axis. It also suggests a gyroscopic vehicle inclination detector for detecting the lateral, as well as longitudinal, tilting of the motorcycle with respect to the perpendicular. As an electric output signal of the detector indicates the lateral tilting of the vehicle in either direction, the interconnected bulb and control lens of the headlamp unit is revolved in the required direction to such an extent that the beam pattern is maintained level about the lamp axis despite the vehicle tipping.
We have found this known beam pattern leveling system unsatisfactory because of the use of the gyroscopic vehicle inclination detector. Operating gyroscopically, the prior art vehicle inclination detector can find the lateral vehicle angle with respect to the perpendicular and not to the road surface on which the vehicle is traveling. For this reason the known leveling system works well only as long as the vehicle is running on a road surface that is horizontal transversely. This limited workability has proved to be very inconvenient because road surfaces are not necessary horizontal in their transverse direction. Curves in highways in particular are usually banked; that is, they tilt laterally upward from the inside edge to the outside edge so as to help vehicles smoothly round the curves without the possibility of being centrifugally carried off the track.
Let us suppose that the motorcycle with the prior art beam pattern leveling system is taking a banked curve, with the vehicle laterally slanting out of the perpendicular and into right angular relationship with the roadbed. Since the vehicle is then at an angle to the perpendicular, the prior art system will detect this angle gyroscopically and correspondingly readjust the beam pattern into an angled position with respect to the bank. Such readjustment is unnecessary or, indeed, undesirable. The beam pattern should have stayed in the level position with respect to the slanting road surface, just as when the vehicle is running an unbanked road.